Sixty-six patients with thymoma have undergone surgical treatment since 1965 and have been assessed from the viewpoint of clinical manifestations and prognosis. Thirty-one patients with encapsulated thymoma were treated with total surgical resection alone, and they had no postoperative tumor recurrence. With the exception of one patient who died of respiratory insufficiency on the fourth day after the operation, 34 patients with invasive thymoma were evaluated on the basis of their postoperative prognosis. Fifteen patients with invasive thymoma died from 1 1/2 months to 10 years, 1 month postoperatively; 9 died of local or metastatic tumor and 6 died of other diseases. Associated autoimmune diseases, as well as the invasive tendency of the tumors, apparently affected the prognosis. Ten-year survival rates of the patients who underwent surgical treatment were as follows: 61.6% for the total group, 74.3% for those with encapsulated thymoma, and 49.4% for those with invasive thymoma. In the surgical treatment for invasive thymomas, one should aim to resect the tumor totally, even though adjacent tissues are resected simultaneously. Even for the patient with total resection of invasive tumor, postoperative radiation should be required. Finally, if residual tumor must be left during the operation, postoperative radiation as well as anticancer chemotherapy should be aggressively scheduled, because postoperative distant metastasis may appear in these patients with residual thymoma.